This invention relates to a lime kiln whose body has a unique top-shape.
Lime kilns known to date may be broadly classified into three types: a bottle-type, vertical shaft-type and rotary-kiln type.
Recently proposed is a doughnut-shaped lime kiln with a rotary bed originating with Calcimatic Limited Company of Canada. ("Japanese Handbook of Gypsum and Lime" published on June 15, l972, pp 506-508 and pp 64-68 of its advertisement column). This proposed lime kiln is characterized by a closed doughnut-shaped rotary bed provided with a movable grate. While making one circulaton of the bed, lumps and granules of limestone carried on the grate pass through a prescribed hot space created by combustion of fuel blown into the kiln through numerous holes drilled in a certain section of a stationary lid; and when the kiln bed makes one rotation, a product of calcined lime is continuously scraped off the terminal point of the rotary bed. One rotation of the kiln bed takes about 50 to 70 minutes.
The above-mentioned Calcimatic Kiln produces as much as 100 to 600 tons of quick lime per day and the raw limestone granules are uniformly calcined regardless of size, though said kiln does not have a merit in thermal efficiency. However, said proposed Calcimatic Kilm still has the drawback that, with accessory equipment included, the kiln requires a tremendously huge apparatus and an extremely large plant area.
One of the present inventors previously disclosed an invention of a unique compact lime kiln of 30 to 50 ton-capacity per day which was designed to be annexed to an existing large lime kiln and efficiently calcine fine granules of limestone unadapted to be handled by such existing large lime kiln. (Japanese Patent Application published before examination with the Number of 104596/78, published on Sept. 11, 1978). This proposed compact lime kiln is of the type similar to the afore-mentioned Calcimatic Kiln in that a horizontal kiln bed is rotated. However, said lime kiln has a top-shape as against the doughnut-shape of the Calcimatic Kiln, and has a simple construction and is easily operated with successfully elevated thermal efficiency.
There will now be described by reference to FIG. 1 the construction of a top-shaped lime kiln embodying the prior invention.
Opened to the underside of the central portion of a platter-shaped bed 4 is a calcined lime-cooling chamber 7 provided at the bottom with a port concurrently used to suck in cooling air which acts as secondary air for complete combustion of fuel, and to allow the discharge of the calcined product. The bed 4 is rotatably supported by means of wheels 3 on a circular rail 2 laid on a trestle 1a. An upper lid section comprises a stationary coaxial round lid 10 covering the upper opening of the platter-shaped bed 4. The lid 10 has a fuel combustion chamber 15 including a downwardly extending burner 14 positioned at the central upper port. The lid 10 has an annular gap 12 defined between the peripheral wall thereof and that of the platter-shaped bed 4. The upper lid section is supported on a trestle 1 fixed on the ground. The annular gap 12 is closed by a gas sealer 13 generally used with a rotary kiln bed, thereby preventing gas generated in the kiln from leaking to the outside. Raw limestone granules 20 run down from a raw material tank 18 set on a trestle 1c through a proper number of chutes 19 into the air-tight annular gap 12. This gap is used to preheat the charged raw material by the passage of kiln exhaust. Calcined lime is scraped off by scrapers 17, each provided with a spring fixed to the lid 10, from the platter-shaped kiln bed 4 into a cooling chamber 7. After being cooled, the calcined lime is continuously taken out of the kiln by a rotary table extruder 8.
The above-mentioned lime kiln of the preceding invention has the following advantages.
(1) The slowly rotatable kiln body has a top-shape, thereby enabling a raw material-preheating region, calcining region and calcined lime-cooling chamber to be set collectively in a compact arrangement, and has a smaller kiln size than any of the prior art kiln bodies, as measured on the basis of the same production capacity. Further, heating in the kiln is effected by a proper combination of radiation and convection, realizing extremely high thermal efficiency. Maintenance and repair of the kiln interior can be easily carried out with the lid lifted by turnbuckles 11 of suspension hooks. Furthermore, a single burner provided at the center of the kiln body allows for easy operation. Since the cooling chamber 7 lies immediately below the burning zone, flames ejected from the burner are uniformly spread through the kiln and completely burned by the preheated secondary air rising from the cooling chamber 7.
(2) The annular gap 12 acts as a zone for preheating the charged raw limestone. This preheating zone has a large volume and a small height. The kiln exhaust slowly rises through said preheating zone with little passing resistance, reducing the power consumption of a blower 23. The rotation of the kiln bed suppresses the possibility of charged raw material hanging in the preheating zone, effectively realizing the uniform preheating of said charged raw material.
(3) Raw limestone granules which are thinly spread on the rotary kiln bed are preferred to have a sufficiently small size for calcination. Where, however, said granules have too small a size, then narrow gaps in between present a high resistance to the passage of kiln exhaust through the preheating zone. Therfore, the raw limestone granules are desired to have a size ranging between 4 and 40 mm. Such small limestone granules have hitherto failed to be used with the existing vertical type lime kiln. When annexed to an existing lime kiln, the above-mentioned top-shaped lime kiln enables crushed limestone granules having a broader range of size than obtained in the past to be easily utilized.
(4) Raw limestone granules thinly spread on the kiln bed need not have a considerably high mechanical strength. Therefore, the lime kiln embodying said preceeding invention allows the use of not only limestone granules but also pellets about 10 mm in diameter which are prepared from fine powders of calcium carbonate or lime cake, a waste material from the industrial manufacture of, for example, beet sugar.
Nevertheless, the top-shaped lime kiln embodying the preceding invention has later been found to be still defective. The facts are that the scrapers 17 provided with a spring do not function effectively; those portions of the scrapers 17 which are directly exposed to the kiln flames often tend to be damaged; and the partially imperfect calcination of raw limestone, as is observed in conventional lime kilns, unavoidably takes place in said top-shaped lime kiln, too.